1. Field of the Invention
The present invention relates generally to a mobile terminal having a camera module and, more particularly, to a mobile terminal and auto-focusing method for the same which enable rapid and accurate auto-focusing through compensation of lens position errors due to characteristics of a voice-coil motor (VCM).
2. Description of the Related Art
Advances in information and communication technologies have enabled a rapid growth in utilization of mobile terminals. Today, advanced mobile terminals support additional functions, such as text message and image transmission, MPEG-1 Audio Layer 3 (MP3) audio file playing, gaming, and photographing.
A mobile terminal having a camera module provides an auto-focusing function. To facilitate the movement of a lens for auto-focusing in a small and thin mobile terminal, a small voice-coil motor (VCM) is preferred as an actuator, instead of the conventional step motor.
FIG. 1 is a graph illustrating a lens position-to-focus value relationship in a conventional mobile terminal. Auto-focusing requires finding the maximum focus value for optimum focus in a focus value curve according to the movement of lens positions. Normally, a hill-climbing search algorithm is used to find this maximum focus value. A conventional hill-climbing search algorithm utilizes a two-stage search process for rapid convergence, as illustrated in FIG. 2.
In the first stage, the lens is repeatedly moved forwards in the unit of a first step size, which represents the distance of lens movement in a single move, until the focus value gradient changes sign. A gradient change occurs when the current focus value after the most recent lens move is less than the previous focus value. Upon detection of the gradient sign change, the second stage begins. In the second stage, the lens is repeatedly moved backwards in unit of a second step size less than the first step size until the focus value gradient changes sign.
In the case where the maximum lens displacement (maximum stroke) is evenly divided by 256, the first step size is +16 (plus for forward lens movement), and the second step size is −2 (minus for backward lens movement), maximum 16 frames are necessary for each search stage. That is, lens movement for auto-focusing may require a time duration corresponding to 32 frames in total.
Compared to a digital camera, a mobile terminal has a smaller lens aperture and requires a longer exposure time (frame time, tframe) to pass a minimum amount of incident light for image formation. The shutter speed of the mobile terminal is 15 frames per second. When a conventional hill-climbing search algorithm is used, a significant time duration of 2.13 seconds would be required for focusing in the mobile terminal.
A voice-coil motor has advantages in terms of a short response time suitable for translation motion, a relatively long stroke suitable for miniaturization, and high-precision position control. However, several parameters such as the lens weight, lens size and lens inertia cause a time delay in the lens movement to a desired target position. In particular, hysteresis characteristics of the voice-coil motor and lens inertia cause a generation of different focus values under identical conditions.
As described above, a direct application of a conventional auto-focusing method to a mobile terminal using a voice-coil motor may cause inaccurate lens focusing.